I can tell it is something bizarre. What I just noticed is, if I checked the call screening option to "On" where I would have to hit 1 when picking up to connect, everything works perfectly! Callers get through to voicemail if I don't pickup and if I do pickup, I hit 1 and it connects to them. My cell phone also rings in addition to my Obi device.
Also, if I turn call screen off, back to original issue right.... but if I uncheck the Obi device, everything works fine, of course Obi device doesn't ring then but linked phone and voicemail on GV number go back to normal. So it seems something about the Obi device being active on the account is messing it up.